The effective spell power granted by this item is affected primarily by cast time. This trinket benefits more from shorter cast times.
The formula for calculating the amount of effective spell power granted by this trinket is:

E = 512 * 10 / (45 + (t / 0.1))

Where

E = effective +spell power
t = cast time

The following Java Program can be used to obtain a table of effective +haste for different cast times.